Charnockite Series Research Articles

The paragenesis of garnet in charnockite, enderbite, and related granulites

SummaryThe presence of garnet in acid rocks of the charnockite series south of Madras is reported. Petrographic deseriptions of garnetiferous enderbites and related pyroxene-granulites, khondalites, and leptynites are accompanied by new chemical analyses of 6 rocks, 8 garnets, and 5 orthopyroxenes from the type area near Madras, and from Ceylon, Varberg, and Uganda. The garnets show a considerably higher FeO/MgO ratio than the coexisting ortbopyroxenes and the host rocks. The presence or absence of garnet appears to be related to the bulk chemistry of the rocks. Basic garnetiferous granulites associated with these rocks are considered to represent an unstable mineral assemblage and may be genetically unrelated to the acid types. Attention is drawn to the prevalence of enderbitic rocks in the type area.

XVIII.—The Geochemistry of the Charnockite Series of Madras, India

SynopsisSince the classical work of Sir Thomas Holland at the beginning of this century on the charnockite series in Peninsular India there have appeared numerous papers describing rocks from all over the world which have been claimed to belong to this series. The rocks from the type area around Madras have now been re-examined, with particular reference to their chemistry and chemical mineralogy, to provide further evidence for their mode of origin.Following petrographic descriptions, fifteen new analyses of these rocks are presented together with trace element determinations, and these are shown to produce smooth curves on a variation diagram. For several rocks all the constituent minerals have been analysed, and it has thus been possible to discuss the mineralogical location of the various major and trace elements in these rocks. Trace element determinations are presented for the 43 minerals analysed together with those for a further 35 minerals not chemically analysed, and their variation within the mineral species is discussed.The possible modes of origin of these rocks are considered, and from the evidence obtained they are held to represent a plutonic igneous rock series which has undergone recrystallization in the solid state on being subjected to plutonic metamorphism.

ARCHEAN GEOLOGY OF THE TURKU DISTRICT IN SOUTHWESTERN FINLAND

The Turku area is situated in the islands and along the sea coast in southwestern Finland. On the northwest it joins the Kalanti area which has been mapped and described earlier (Hietanen, 1943). The oldest rocks are kinzigites (cordierite-garnet gneisses) and amphibolites, both highly metamorphosed and migmatized. Kinzigites are of sedimentary origin and show bedding locally. Amphibolites are volcanic or sedimentary as are also the finer-grained leptites which occur with them. The intrusive rocks are grouped as follows: (1) intrusives of the first cycle, (2) intrusives of the second cycle, and (3) the post-tectonic intrusives. (1) The oldest intrusives occur as concordant oblong bodies in the oldest rocks. The acidic members are poor in potash, belonging to the trondhjemite series. Intermediate and basic rocks are members of either trondhjemite or charnockite series. Texture is granoblastic. Remnants of older rocks and basic inclusions are common. In small bodies lineation and cleavage parallel the same structures in the country rocks. The cores of the larger plutons are structureless or show a vertical lineation. Many occurrences among charnockitic and trondhjemitic rocks are clearly formed metasomatically or metamorphically. The bulk of these rocks, however, show features common in igneous rocks. (2) The intrusives of the second cycle (the granite series) are rich in potash and penetrate the older rocks concordantly and also disconcordantly. The structures of the narrow bodies parallel the walls; the centers of the larger ones are structureless. (3) The youngest intrusives in the area are diabase dikes, anorthosite, and rapakivis. Migmatization and granitization of the oldest rocks are a result of joint action of long duration of (1) introduction of feldspars and quartz, and (2) metamorphic differentiation. The processes started contemporaneously with the folding and emplacement of the oldest intrusives and continued after the folding until emplacement of pegmatites of the red microcline granite series.

Enderbite, A New Member of the Charnockite Series

The term “charnockite series” is now commonly in use for a group of genetically related rocks ranging from ultrabasic to acid members having the chemical and mineralogical features of the rock suite of the type area of Madras (Holland, 1893, 1900). The acid member “charnockite” is defined by Holland (1926) as “essentially a hypersthene granite…composed of hypersthene, microcline, quartz, and iron ores…”.

The Charnockite Series of Uganda, British East Africa

I. Introduction Though Sir Thomas Holland (1900, p. 131) counselled that the designation “charnockite series” should not be applied to apparently similar rocks outside India unless their genetic relationship with the type series could be proved, the striking parallelism between that series and the rocks to be described in this paper appears to justify reference to the latter as “the Uganda chamockite series”. The Uganda series comprises four main divisions—acid, intermediate, basic, and ultra-basic—and includes nearly all the types known in India. In Uganda, as in India, the sub-acid to basic types predominate. Hypersthene-bearing rocks were discovered at several localities in Bunyoro by E. J. Wayland and W. C. Simmons, and subsequently in Karamoja by Wayland and in the West Nile District by Simmons. A few of these rocks were provisionally identified in the field as “charnockites”, but the existence of a series of rock types akin to the charnockite series of India was not established until 1931, when I was able to make a complete petrological study of the collections. Hypersthene-bearing rocks were then found to be more abundant and more widely distributed than had hitherto been suspected, their occurrence being discovered in many new localities in each of the three main areas just mentioned. The present paper is based on a study of 110 specimens of the charnockite series, 10 representative rock analyses, nine analyses of minerals, and a large number of specimens of country rocks. II. Distribution and Mode of Occurrence The outcrops of the charnockite series

Petrology and the Western Rift of Central Africa

IN the course of several journeys between Masindi and Butiaba, and one from Butiaba to Hoima (via Waki Camp), specimens of the various gneisses were collected from roadside exposures. Thin sections of these were subsequently studied by the writer in England with the result that they were found to yield evidence of tectonic significance. In addition, the writer has, during the last three years, examined petrographically a large number of other rocks from those parts of Uganda bordering the Lake Albert Depression. In particular he has made a special study of the Charnockite Series, which are widely spread over the northern half of the Protectorate, with the result that it has been possible to observe the effects of the rift valley movements on these rocks in the regions adjacent to Lake Albert. It is hoped to publish a petrological and geochemical study of the Charnockite Series of Uganda in the near future. In the following pages it is proposed in Part I to present petrological evidence from the basement rocks of various districts in the hinterland of the Lake Albert scarps and then in Part II to point out the tectonic significance of this evidence in the light of the various theories of rift valley formation. In Part III the relation between the petrological characters of the volcanic rocks and the rift valley tectonics will be briefly reviewed. In the case of the basement rocks the remarks will be confined to the Lake Albert Depression, but in the case of the volcanic rocks the discussion will extend also to Lakes George and Edward and the Birunga volcanic field.

The Crystalline Metamorphic Rocks of South-Western Togoland

1. A belt of gneiss of dioritic composition lies along the west side of the Togo Range, and is exposed at intervals practically all the way from Pode to Atakpame.2. Associated with it are hornblende- and mica-schists, garnet-amphibolites, and hypersthene-gneisses resembling rocks of the charnockite series. These last form the greater part of Agu Mountain, and also occur at several places along the foot of the Togo scarp.3. The valley of the Dsawoë River appears to be in large part underlain by gabbro and dolerite types of rock, with associated dyke rocks.4. A tract of garnetiferous biotite-gneiss, partly granitic in composition, occupies that part of the basin of the Amu River due south of Atakpame. It is associated with schists similar to those accompanying the dioritic gneiss.5. Basic intrusions occur in the gneisses, e.g. serpentine at Djeti and pyroxenite north of Pode. A pyroxenite north of Akata is intrusive in rocks of the Togo series. No granitic intrusions were found.6. The rocks described form a basement upon which the rocks of the Togo series and other younger ones rest.

The charnockite series of igneous rocks

The charnockite series of igneous rocks

VII.—Contributions to the Geology of Ceylon: III. The Balangoda Group

The name ‘Balangoda group’ is proposed for a series of granitic and pegmatite-like rocks, intrusive in, but distinct from, the Charnockite series; first met with in the Balangoda district, but evidently widely distributed over a large area between Balangoda and Hatton. The rocks are best described as granites, but occur most often in rather narrow dykes, after the manner of pegmatites. Yet there is no reason for separating the smaller from the larger masses, and the term granite is applied to both. The group (of which a more detailed account will ultimately be needed) includes in particular zircon granite, allanite granite, magnetite granite, and granite without conspicuous accessory minerals; as well as the probably similar rocks in which the hitherto unlocated minerals geikielite, baddeleyite, rutile, fergusonite, thorite, thorianite, etc., may be looked for; and the vein of pegmatite at Gampola, which consisted of quartz, felspars, and biotite, with apatite, ilmenite, tourmaline, and the new mineral described as thorianite as accessory minerals.These granites are intrusive in the Charnockite series, and though frequently occurring in lenticular masses (Denagama) with a disposition parallel to that of the foliation planes of the charnockites, have often been observed to transgress these foliation planes and to behave as intrusive rocks. Contact phenomena have not, however, been observed, except perhaps in a slight tendency to a peripheral fineness of grain in the intrusive rocks. At the junctions granite and charnockite are usually welded together, there being no absolutely hard line of separation, although the junction may be called sharp; in the case of the larger masses no good junctions have been seen.

The Point-de-Galle Group (Ceylon): Wollastonite-Scapolite Gneisses

I.Introduction. Prof. A. Lacroix has recorded the existence of wollastonite- and scapolite-bearing rocks in Ceylon. The specimens examined by him can hardly, however, have come from Galle, as they contain garnet, a mineral never found in the rocks of the Galle Group described below. It seems therefore probable that rocks allied to those of Galle, but differing from them in petrological details, will be found elsewhere in Ceylon. I have on a former occasion given a short description of the Galle rocks, and am now able to supply a more detailed account of the rocks and to describe their field-relations. There are excellent exposures at Galle, within the Fort on the western side, between the Signal flagstaff and the north-western bastion ; outside the Fort below the north-western bastion ; and south-west of the Victoria Park near the remains of an old powder-magazine. All these exposures can be easily visited in the course of one whole day spent at Galle. II. General Description. The Galle rocks differ from normal types belonging to the Charnockite Series in the following ways :— (1) The presence of wollastonite, scapolite, and sphene. (2) The existence of definite dykes and segregation-veins crossing the foliation. (3) The absence of garnet, hypersthene, mica, and original hornblende, and bands of ‘granular quartz-rock’. (4) Somewhat coarser grain. But resemble them closely in (5) The variability of chemical and mineralogical composition.